A. Field of the Invention
The Invention relates to positioning and securing a work piece for accurate shaping of the work piece using computer numerical controlled (CNC) machines. Specifically, the Invention relates to the adaptive positioning and securing of an imprecise work piece, such as a casting, forging or composite layup, in a non-adaptive fixture where the fixture may be repeatably attached to the table or stage of one or more CNC machines.
B. Statement of the Related Art
As used in this document, a ‘computer numerical controlled machine,’ or ‘CNC machine,’ refers to a manufacturing apparatus having automated control of any shaping, subtractive or additive manufacturing process that describe a series of movements, and includes milling, turning, drilling, grinding, electric discharge machining, laser cutting, water jet cutting, welding, friction stir welding, ultrasonic welding, flame cutting, plasma cutting, bending, spinning, punching, pinning, gluing, fabric cutting, sewing, tape and fiber placement, routing, picking and placing, sawing and 3D printing. As used in this document ‘CNC machine’ also refers to any other automated manufacturing technology that utilizes a series of movements of a work piece or of a tool with respect to a work piece or of a work piece and a tool with respect to each other for the purpose of changing the shape of the work piece.
As used in this document, the term ‘work piece’ means the object to be shaped from an imprecise, rough condition to a finished machined object.
In a manufacturing operation utilizing a CNC machine, the work piece, the tool or both will move in a precisely described path under the control of a computer. The accuracy of the finished manufactured part relies on the accuracy of the movement of the tool and also relies upon the accuracy of the location of the work piece with respect to the tool. For a work piece that has imprecise dimensions, the dimensions of the work piece vary from work piece to work piece, making the repeatable production of precision manufactured parts problematic. Imprecise, variable work pieces are common in industrial casting and aerospace manufacturing and commonly include castings, forgings, and layups of a composite material, such as carbon fiber composite.
The problem is that each imprecise work piece defines unique manufacturing constraints to successfully produce the finished part; namely, each edge and surface of the finished part must fall within the volume defined by the work piece. In addition, the work piece may define additional constraints relating to manufacture or operation of the finished part, such as local internal changes in material specifications or strand orientation in the case of the carbon fiber composite. Each of those constraints limits the permissible locations and orientations of the work piece with respect to the tool that will result in high-quality finished parts and that will avoid rejected parts and waste.
The prior art reflects two approaches to the problem of the variable work piece: ‘adaptive machining’ and ‘adaptive fixturing.’ In adaptive machining, an imperfect work piece is first measured. The work piece is then attached to the CNC machine in the same orientation as each prior and each succeeding work piece. Due to variation between work pieces, the programmed movement of the tool of the CNC machine must be changed, or ‘adapted,’ for each work piece to conform to the measurements and other constraints of each work piece. A major disadvantage of adaptive machining is that the location of the work piece must be identified and the programming of the CNC machine must be changed with every succeeding work piece, with the resulting delay and opportunity for error. In addition, a typical machining operation for a complex manufactured part may require multiple sequential steps performed on multiple CNC machines. With each transfer of the work piece to a new CNC machine, the location and orientation of the work piece must be re-identified with respect to the tool and the programming of the CNC machine must be changed, multiplying the time required and opportunities for error by the number of CNC machines involved and reducing the capabilities of the system to those of the least capable CNC machine. Another disadvantage is that many CNC machines are not capable of the six degrees of freedom required to take full advantage of adaptive machining.
In adaptive fixturing, the work piece is attached to a movable fixture that is attached to the table or stage of the CNC machine. The work piece is measured at several locations. The work piece is moved by moving the adaptive fixture until the work piece is in a desired location and orientation with respect to the tool to ‘adapt’ for the imprecise shape of the work piece. The adaptive fixture is locked in position and the CNC machine activated to shape the work piece.
Like adaptive machining, adaptive fixturing has several deficiencies. First, the combination of the CNC machine and the adaptive fixture typically must be customized to a particular type of work piece and thus have very significant monetary and time costs associated with their development, as well as the loss of flexibility since the CNC machine and adaptive fixture combination cannot be used for other types of work pieces. Second, adaptive fixtures tend either to require tedious and time-consuming manual adjustment or to be bulky electromechanical devices that cannot be easily moved between CNC machines. The use of adaptive fixtures restricts the flexibility of the CNC machines and significantly increases cost, particularly when using adaptive fixturing on work pieces that require multiple processes on multiple CNC machines. Third, adaptive fixtures cannot comprehensively measure a part, which means that process adaptations cannot be based on areas of the part that are inaccessible for measurement when the part is in the work-holding position. Finally, due to prohibitive costs and complexity, adaptive fixtures typically do not have the full six degrees of spatial manipulation necessary to correct for certain positioning and manufacturing imperfections.
The prior art does not teach the work-holding system and method of the Invention.